从Oracle Varchar2中查找和删除非ascii字符

We are currently migrating one of our oracle databases to UTF8 and we have found a few records that are near the 4000 byte varchar limit. When we try and migrate these record they fail as they contain characters that become multibyte UF8 characters. What I want to do within PL/SQL is locate these characters to see what they are and then either change them or remove them.We are currently migrating one of our oracle da